Giant black holes in the centers of galaxies grow mainly as a result of intergalactic collisions, according to results presented by a group of astronomers led by Dr. Ezequiel Treister from UH Mānoa's Institute for Astronomy, published in the March 25 issue of the international journal Science.

As gas clouds in galaxies are sucked into the central black hole, they emit vast amounts of radiation, giving rise to objects that astronomers call quasars. "We find that these growing black holes are originally hidden by large amounts of dust, but after 10-100 million years this dust is blown out by the strong radiation pressure, leaving a naked quasar that is visible in optical wavelengths and keeps shining for another 100 million years," said Treister.

For this study the group combined data obtained with the Hubble, Chandra and Spitzer space observatories to identify a large number of obscured, dust enshrouded quasars at very large distances, up to 11 billion light-years away when the Universe was still in its infancy. "For many years, astronomers believed that these sources were very rare. Now we are seeing them everywhere!" Treister added. Because most of the emission from these obscured quasars is hidden, astronomers looked at infrared wavelengths, for signs of very hot dust, and in X-rays, which are less affected by obscuration. The investigators discovered that the number of obscured quasars relative to the unobscured ones was significantly larger in the early Universe than it is now.

"We knew theoretically that the mergers of massive gas-rich galaxies were more frequent in the past; these observations fit very nicely within this scenario," added professor Priyamvada Natarajan of Yale University, the second author and theorist on the team.

"We knew that this is definitely the case for nearby galaxies, but this result shows that this happens across the Universe," said UH Mānoa professor and investigation participant David Sanders.

Researchers further analyzed images of these distant galaxies taken by the Hubble Space Telescope, using the new Wide Field Camera 3 installed 10 months ago during the last servicing mission. Theseimages revealed obvious signatures of interactions and mergers, thus confirming the hypothesis of this group. Finally, using a simple theoretical prescription, the authors estimated that it takes about 100 million years for radiation from the growing black hole to wipe out the surrounding dust and gas and reveal the naked quasar.

Major galaxy mergers are important to trigger star formation episodes and modify galaxy morphologies. "This work confirms that mergers are also critical for the growth of the nuclear giant black hole", saidNatarajan. Mergers are therefore essential for the evolution of a galaxy and also cause their central black hole to gain weight during both the obscured and unobscured phases.

Science Express will publish the paper presenting these results on March 25. Ezequiel Treister is an Einstein postdoctoral fellow at the University of Hawaiʻi at Mānoa. Other coauthors of this work are Priyamvada Natarajan (Yale), David Sanders (University of Hawaiʻi at Mānoa), Meg Urry and Kevin Schawinski (Yale) and former University of Hawaiʻi's graduate student Jeyhan Kartaltepe (NOAO).

Figure 1: Examples of strongly-interacting/merging galaxies containing a heavily obscured growing supermassive black hole observed by the Hubble Space Telescope. The top panels show nearby galaxies at ~500 million light-years from us, while the more distant galaxies in the bottom panels are ~ 6.5 billion light-years away, when the Universe was half its current age. For the first 10-100 Myrs after the merger, thegrowing black hole remains highly obscured, after which it becomes an optically bright quasar that shines again for another 10-100 Myrs before it likely reaches its upper limit.

Figure 2: Artistic representation of the quasar stages after a major galaxy merger. Initially, the growing black hole is completely enshrouded by large amounts of gas and dust. After about 100 Myrs the strong pressure provided by the quasar emission is enough to blown out most of the surrounding gas, thus unveiling a "naked" quasar, which is clearly detectable at optical and UV wavelengths. These bright quasars shine for another 10-100 Myrs. Most of the black hole growth happens during the quasar stage. While the black hole before the quasar phase has a mass of about 1 million solar masses, the final mass can be up to 10,000 times larger.